PAN hollow fiber composite membrane

PAN hollow fiber composite membrane

Desalination 280 (2011) 252–258 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m ...

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Desalination 280 (2011) 252–258

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Dehydration of ethyl acetate aqueous solution by pervaporation using PVA/PAN hollow fiber composite membrane Hai-Kuan Yuan a,⁎, Jie Ren a, Xiao-Hua Ma b, Zhen-Liang Xu b a b

College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, Zhejiang Province, China Chemical Engineering Research Center, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 6 May 2011 Received in revised form 3 July 2011 Accepted 5 July 2011 Available online 6 August 2011 Keywords: Polyvinyl alcohol (PVA) Pervaporation (PV) Hollow fiber composite membrane Ethyl acetate (EAc)-water solution

a b s t r a c t Using poly(vinyl alcohol) (PVA) as coating material, tartaric acid (Tac) and maleic anhydride (Mac) as crosslinking agents and the poly(acrylonitrile) (PAN) hollow fiber ultrafiltration membrane as support layer, the PVA/PAN composite membrane was prepared by dip-coating method used for pervaporation (PV) dehydration of ethyl acetate (EAc)/H2O solution. The PVA/PAN composite membrane was characterized by FT-IR spectra and SEM. The effects of cross-linking agents on the swelling degree and PV performance of PVA/PAN membrane were investigated, respectively. The separation factor of PVA/PAN composite membrane increased while the permeation flux decreased with increasing the contents of cross-linking agents, and PVA/PAN membrane exhibited better PV performance when Tac/PVA mass ratio was 0.2. Furthermore, the PV performance of PVA/PAN composite membrane for EAc/H2O solution was investigated with respect to the feed temperatures and feed water concentrations. For PVA/PAN composite membrane with Tac/PVA mass ratio of 0.2, its performance for 2 wt.% water of EAc solution at 40 °C was evaluated as follows: the separation factor and permeation flux were 7270 and 34.5 g m−2 h−1, respectively. In addition, PVA/PAN composite membrane was also used for PV dehydration of EAc/ethanol (EtOH)/H2O ternary solution. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Poly (vinyl alcohol) (PVA), a hydrophilic polymer, is often used in pervaporation (PV) separation of water-solvent mixtures for its good chemical stability and low manufacturing cost [1–3]. However, it is easy to swell in aqueous solution, which leads to the change of the membrane structure and the decrease of the mechanical strength and the permselectivity. To improve the stability and permeation properties of PVA membrane, some methods, such as cross-linking [4–10], grafting [11,12], heating treatment [9,13] have been used. Cross-linking is a commonly used way to stabilize PVA membranes, which can remarkably influence the permeability or selectivity of membrane. Ethyl acetate (EAc) is a very important solvent in the chemical industry, and it is widely used in producing perfumes, plasticizers, varnishes, synthetic resins and adhesive agents. The production of EAc is commonly based on a classical esterification process of acetic acid (HAc) with ethanol (EtOH) in conventional industry [14,15]. The resultant EAc contains the produced water and residual EtOH, which can form binary and ternary azeotropes. Thus, it is difficult to purify EAc just through the conventional energy-intensive distillation [16,17]. PV is a type of energy saving and environmentally friendly

⁎ Corresponding author. Tel.: + 86 571 88320208. E-mail address: [email protected] (H.-K. Yuan). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.07.002

membrane technology with potentially wide separation of azeotropic and close-boiling mixtures, dehydration of solvent, and recovery of organic component in aqueous or organic mixture [18–21]. Among these applications, the PV dehydration for aqueous EAc azeotropes has important commercial benefits. Removal of EAc from aqueous solution is of interest for recovery of solvents and for treatment of wastewater. Hasanoglu et al. [22] used poly(dimethylsiloxane) (PDMS) membrane for PV separation of organics from EAc/EtOH/H2O mixtures with water content above 90 wt.%, and PDMS membrane was much more selective to EAc than to the other components. Tian et al. [23] and Zhu et al. [24] used separately the poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) membrane to separate EAc from its aqueous solutions. Nguyen et al. [25] prepared PDMS dense membranes cross-linked in different conditions for PV separation of EAc-water mixtures, and also reported the influence of cross-linking conditions on the membrane swelling and the selectivity of solvent sorption. However, only few literatures reported the PV dehydration for EAc-H2O system. Xia et al. [26] used PVA/ceramic composite membrane for PV dehydration of EAc-H2O mixtures, and the composite membrane exhibited good PV performance. Yuan et al. [27] prepared perfluorosulfonic acid-tetraethoxysilane/poly(acrylonitrile) (PFSA-TEOS/PAN) hollow fiber composite membrane for PV dehydration of EAc-H2O solution, and the swelling of PFSA in EAc aqueous solution was inhibited by the addition of TEOS. PV dehydration of EAc/EtOH/H2O azeotrope using chitosan/poly(vinyl

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pyrrolidone) (CS/PVP) blend membrane cross-linked by glutaraldehyde (GA) was also reported by Zhang et al. [28], and the CS/PVP membrane with 10 wt.% PVP exhibited excellent PV properties. More previously, only few researchers [29–32] reported PV dehydration for EAc-H2O system through PVA membranes. Usually, the dense or composite flat membranes were used in PV dehydration for EAc aqueous solution, while the hollow fiber composite membranes were rarely reported. In the present work, PVA/PAN hollow fiber composite membrane cross-linked by tartaric acid (Tac) and maleic anhydride (Mac) were prepared by dip-coating method and used for PV dehydration of EAc/H2O binary solution. The swelling degree of cross-linked PVA membrane in EAc aqueous solution was investigated. The PV performances of PVA/PAN composite membrane were studied with respect to Mac or Tac content, feed temperature and feed water concentration for the EAc/H2O binary solution. In addition, PVA/PAN composite membrane was also used for PV dehydration of EAc/EtOH/H2O ternary solution. 2. Experimental 2.1. Materials and instruments Poly(vinyl alcohol) (PVA) with polymerization degree of 1750 ± 50 and alcoholysis degree of 98%, maleic anhydride (Mac) (C. P.), ethyl acetate (EAc) (A. R.) were provided by Shanghai Chemical Reagent Co., Ltd. (China). Tartaric acid (Tac) (A. R.) was obtained from Shanghai Feida Trade Co., Ltd. (China). All other solvents and reagents were purchased from Shanghai Chemical Reagent Co., Ltd. (China). Poly(acrylonitrile) (PAN) hollow fiber ultrafiltration membrane with pure water flux of 14 L m −2 h −1 Bar −1 was self-made in laboratory. Gas chromatograph (GC7890 T, China) equipped with a thermal conductivity detector (TCD) and a GDX-102 packed column, Fourier transform infrared (FT-IR) spectrometer (Nicolet 6700, America) and scanning electron microscope (SEM) (JEOL, JSM-6360 LV, Japan) were used. 2.2. Preparation and characterization of PVA/PAN hollow fiber composite membrane

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tissue paper and weighed immediately. The weight of each sample was monitored periodically until a constant weight was achieved. Swelling degree of membrane is calculated as follows: SD =

Ws −Wd × 100% Wd

ð1Þ

where Wd and Ws are the weight of dry and swollen membrane, respectively, (g). 2.4. PV process of PVA/PAN composite membrane Fig. 1 illustrates the schematic diagram of the PV setup. The hollow fiber membrane cell has three modules, and each module consists of 3–5 fibers with lengths of 20 cm. The permeate side was maintained at a vacuum pressure (2.6 kPa) using a vacuum pump. When the steady state was reached, the permeate was collected in traps cooled by liquid nitrogen. The composition of the permeate and the feed were analyzed by gas chromatograph. The separation performances of PVA/PAN composite membrane are characterized in terms of permeation flux (J) and separation factor (α). Permeation flux (J, g m −2 h −1) is defined as follows: J=

W t×A

ð2Þ

where W is the total amount of permeate (g), t is the experimental time interval (h), and A is the total outer surface area of hollow fiber membranes (m 2). The separation factor (α) is defined as follows: α=

ðYW =YEAc Þpermeate ðXW =XEAc Þfeed

ð3Þ

where YW, YEAc, XW and XEAc are the weight fractions of water and EAc in the permeate and in the feed, respectively. 3. Results and discussion

Firstly, PVA was dissolved in deionized water and stirred at 90 °C for 6 h to form 5 wt % of PVA coating solution. Prescribed amount of Mac or Tac was added into the PVA aqueous solution with few drops of hydrochloric acid (HCl) catalyst, and then the mixture was stirred for 2 h. The insoluble impurities were removed using a glass filter, and the coating solution was obtained after being degassed. Secondly, PAN support membranes were immersed in the coating solution for 30 s, then taken out and dried at room temperature for more than 12 h. In order to make the coating layer more homogeneous, every fiber was coated twice. After coated for the second time, the membranes were hung inversely when being dried. Finally, the PVA/PAN hollow fiber composite membranes were obtained after being treated at 60 °C for 24 h in a vacuum oven. The outer surface and cross section of PVA/PAN composite membrane were visualized by SEM. The cross section of hollow fiber was prepared after breaking the fiber in liquid nitrogen. All samples were sputter-coated with gold before analysis.

3.1. Morphology of PVA/PAN composite membrane As shown in Fig. 2, PVA/PAN composite membrane consists of two layers, the PAN support layer and the PVA selective layer, and these two layers integrate tightly. PAN ultrafiltration membrane has dilayer finger-like structure and rough outer surface, while the outer surface of PVA/PAN composite membrane becomes smooth and dense. The

2.3. Swelling degree of PVA membrane In order to measure the swelling degree of PVA/PAN composite membrane in EAc aqueous solution more accurately, PVA homogeneous membrane was prepared on a clean glass plate by casting method, and the treatment process was the same with the PVA/PAN composite membrane. The dry membrane was immersed in 2 wt.% water of EAc solution at room temperature in a closed glass flask. After being taken out from the solution, the swollen membranes were wiped rapidly with

Fig. 1. Schematic diagram of the PV process setup. 1. temperature controller; 2. thermocouple; 3. heater; 4. water bath; 5. feed solution tank; 6. membrane module; 7. vacuum valve; 8. cold trap; 9. stopcock; 10. manometer; 11. absorbed column; 12. buffer; 13. vacuum pump.

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Fig. 2. Surface and cross section morphologies of PAN hollow fiber support membrane and PVA/PAN composite membrane. A-1, A-2: cross section of PAN membrane; A-3: outer surface of PAN membrane; B-1: cross section of PVA/PAN composite membrane; B-2: outer surface of composite membrane.

coating layer with the thickness of about 15 μm is clearly found on the outer surface of PAN membrane, which is almost nonporous.

which also suggests that the cross-linking reactions occur between the hydroxyl group in PVA and carboxyl group in cross-linking agents [35].

3.2. FT-IR analysis of selective layer of PVA/PAN composite membrane

3.3. Chemical cross-linking analysis of Mac and PVA

PVA and cross-linked PVA selective layer was characterized by FT-IR spectra, as shown in Fig. 3. The distinct broad adsorption bands at 3000–3600 cm−1 correspond to the structural hydroxyl groups [33]. Compared to FT-IR spectra of PVA, the relative intensity of the hydroxyl bands near 3300 cm−1 of cross-linked PVA membranes decreases, which indicates that the number of hydroxyl groups in PVA membrane decreases after being cross-linked by Mac or Tac. The sharp band of 2900 cm−1 corresponds to CH group [34]. The peaks at 1706 and 1730 cm−1 are attributed to the stretching vibration of carbonyl group in FT-IR spectra of PVA cross-linked by Mac and Tac, respectively,

In preparation of PVA based membrane for dehydration of solvent, the cross-linking agent plays an important role to get the desired performance, which makes membrane resistant to the feed and the permeates [4,36]. Therefore, it is necessary to know the chemical cross-linking mechanism between cross-linking agent and PVA. The structure of tartaric acid (Tac) is

. The

maleic anhydride (Mac) dissolves in water and generates maleic acid, and the structure of maleic acid is

. Tac

has a similar structure to maleic acid, just two hydroxyl groups more are in Tac than in maleic acid. Therefore, they have the same chemical cross-linking mechanism with PVA. There are three kinds of cross-linking reactions between hydroxyl groups in PVA and carboxyl groups in Mac, that is, intra-chain (a), inter-chain (b) and single-esterification (c) [37], as shown in Fig. 4. The chemical cross-linking between PVA and Mac results in the decrease of the free hydroxyl groups, which leads to the decrease of hydrophilicity of PVA membrane. Thus, the swelling of PVA in aqueous solution can be reduced or inhibited through chemical cross-linking. 3.4. Swelling characteristics of PVA membrane

Fig. 3. FT-IR spectra of PVA and selective layers of cross-linked PVA/PAN composite membranes.

Fig. 5 shows the effect of cross-linking agent content on the swelling degree of PVA membranes in 2 wt.% water of EAc solution. The swelling degree of PVA membranes in EAc aqueous solution decreases as Tac or Mac content increases, as shown in Fig. 5. The degree of chemical cross-linking between PVA and Mac or Tac is influenced by the contents of cross-linking agents. The higher the degree of Mac cross-linking reaction, the smaller the number of free hydroxyl groups and the lower the hydrophilicity of PVA membrane. The chemical cross-linking makes the structure of PVA membrane denser, consequently, the mobility of polymer chains and the affinity between water and PVA decrease. In many cases, the weaker the affinity between the permeates and the membrane is, the lower the

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Fig. 4. Schematic diagram of cross-linking reaction between Mac and PVA.

swelling degree of the membrane [38]. Compared with the PVA membrane cross-linked by Mac, the PVA membrane cross-linked by Tac has less swelling agree in the entire range of cross-linking agent/PVA mass ratio, as shown in Fig. 5. This is because there are more hydroxyl groups in Tac structure than those in Mac, and the degree of chemical cross-linking between Tac and PVA is higher with the same content of cross-linking agents.

3.5. PV performance of PVA/PAN composite membrane 3.5.1. Effect of cross-linking agent content on PV performance of PVA/PAN composite membrane The PV performance of membrane is closely related to its structure, and the cross-linking is an efficient way to modify the structure of PVA membrane. The effect of the cross-linking agent content on the PV performance of PVA/PAN composite membrane for 2 wt.% water of EAc solution at 40 °C is shown in Fig. 6. With increasing the contents of cross-linking agents, the separation factors of PVA/PAN composite membrane increase, while the permeation fluxes decrease, which is attributed to the higher degree of the chemical cross-linking between Tac or Mac and PVA. For instance, PVA treatment with more maleic acid will mainly lead to inter-chain cross-linking, shown in Fig. 4(b), which forms the higher cross-linking density in PVA membrane [39].

Fig. 5. Effect of cross-linking agent/PVA mass ratio on the swelling degree of PVA membranes.

The formed network and the more compact structure in cross-linked PVA result in the decrease of the free volume of membrane. Due to the decreased free volume and the increased chain stiffness of PVA membrane, the diffusion resistance of EAc through the membrane increases. Therefore, the permeation flux of PVA/PAN composite membrane decreases and the separation factor increases after being cross-linked. As also shown in Fig. 6, the separation performance of PVA/PAN membrane cross-linked by Tac is better than that of PVA/PAN membrane cross-linked by Mac when using the same content of Mac or Tac. When the mass ratio of Tac/PVA is more than 0.2, the separation factor of PVA/PAN membrane increases, while the permeation flux decreases sharply. Therefore, the suitable mass ratio of Tac/PVA is chosen as 0.2.

3.5.2. Effect of feed temperature on PV performance of PVA/PAN composite membrane In general, with increasing the feed temperature, the permeation flux of membrane increases and the separation factor decreases. Fig. 7 shows the effect of the feed temperature on PV performance of PVA/PAN membrane cross-linked by Tac for dehydration of 2 wt.% water of EAc solution. As shown in Fig. 7, the permeation flux of PVA/PAN composite membrane increases slightly from 30 to 50 °C, and it increases remarkably from 50 to 60 °C. However, the separation factor first increases with increasing feed temperature below 50 °C, and then decreases afterwards. The increase of permeation flux with feed temperature can be explained traditionally by the increase of mobility of the polymer chains and the expansion of the free volume [40]. Theoretically, the selectivity of membrane would decrease because of increases in permeabilities of both EAc and water. However, both the permeation flux and the separation factor increase simultaneously with feed temperature from 30 to 50 °C, which is in contrast to the typical behavior of polymer membranes. With increasing feed temperature, the interactions between EAc and water weakens, and the diffusing components have greater driving force. As the permeability of water increases more rapidly than EAc with feed temperature, the separation factor will increase [41,42]. Whereas, when the feed temperature is higher than 50 °C, the coupling flux affect is more evident and predominant [9,40], which results from the increases of the mobility of polymer chain and the permeant solubility in membrane, therefore, the separation factor will decrease.

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Fig. 6. Effect of cross-linking agent/PVA mass ratio on PV performances of PVA/PAN composite membranes.

3.5.3. Effect of feed water concentration on PV performance of PVA/PAN composite membrane Fig. 8 shows the effect of feed water concentration on PV performance of PVA/PAN composite membrane at 40 °C. The feed concentration ranges from 0.5 to 3 wt.% water of EAc solution, which is chosen by considering the azeotropic mixture of EAc and water with miscible phase occurring at compositions less than 3 wt.% water. It is observed that the permeation flux of membrane increases, whereas the separation factor decreases gradually as the feed water concentration increases. A higher feed water concentration corresponds to more water molecules sorbed into PVA, as a consequence, the hydrophilic PVA layer is more swollen [43]. This makes both water and EAc molecules require less diffusion resistance through the membrane, thus, the total permeation flux increases and the separation factor decreases. 3.6. PV dehydration of PVA/PAN composite membrane cross-linked by Tac for EAc/EtOH/H2O ternary solutions As mentioned above, PVA/PAN composite membrane has good dehydration performance for EAc/H2O binary solution. When producing EAc by reaction distillation, the coarse top products are mainly EAc/EtOH/H2O ternary azeotrope. Therefore, the additional dehydration experiments were performed to study PV performance of PVA/PAN composite membrane for EAc/EtOH/H2O (90/2/8 (wt.%)) ternary solution, which was from Wujing Chemical Co., Ltd. (China).

Fig. 7. Effect of feed temperature on PV performance of cross-linked PVA/PAN composite membrane.

Fig. 9 shows the effect of the feed temperature on PV performance of PVA/PAN composite membrane for EAc/EtOH/H2O ternary solution. As shown in Fig. 9, The total permeation flux (JT) increases from 133.6 to 182.2 g m −2 h −1 with feed temperature from 30 up to 60 °C, and the permeate water concentration decreases from 98.3 to 94.0 wt.%, which indicates that the cross-linked PVA/PAN composite membrane also exhibits good dehydration performance for EAc/EtOH/H2O ternary solution. The traditional trade-off behavior between an increase of permeation flux and a decrease of selectivity are observed in Fig. 9 as the feed temperature increases, which is different from that for EAc/H2O binary solution shown in Fig. 7. Except for the water, the polarity of EtOH is stronger than EAc, and PVA has a good affinity towards EtOH rather than EAc, thereby, the selective permeation rate of EtOH through PVA should be higher than that of EAc. However, it is interesting to find that the EAc concentration is higher than that of EtOH in the permeate. The driving force of PV operation is chemical potential differences between two sides of the membrane [44]. The pressure driving force is equal due to the same operating process. The EtOH concentration in the feed is very low, therefore, the high EAc concentration increases the driving force for PV and consequently increases the permeate amount of EAc, as shown in Fig. 9. The temperature dependence of the permeation flux (J) can be expressed by the Arrhenius equation, J = A0 expð−ΔEa = RT Þ

ð4Þ

Fig. 8. Effect of feed water concentration on PV performance of cross-linked PVA/PAN composite membrane.

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Fig. 9. Effect of feed temperature on PV performance of PVA/PAN composite membrane for EAc/EtOH/H2O ternary solution.

where A0 is the pre-exponential factor, ΔEa is the apparent activation energy of permeation, and T is the operating temperature. Arrhenius plots for PVA/PAN composite membrane are shown in Fig. 10. The relationships between the total and partial permeation fluxes and the reciprocal of temperature (T −1) indicate a good linearity in the given temperature range. The apparent activation energy (ΔEa) indicates that the feed temperature affects the permeation behaviors of components in membrane. The apparent

activation energy of total permeation flux and partial fluxes of H2O, EtOH, EAc for PVA/PAN composite membrane calculated from the slopes of Arrhenius plots are 9.08, 7.81, 24.47 and 51.72 kJ mol −1, respectively. Therefore, the permeation priority of components through PVA/PAN composite membrane is H2O N EtOH N EAc. 4. Conclusions Dehydration of EAc aqueous solution by PV using PVA/PAN hollow fiber composite membrane prepared by dip-coating method was described. The content of Mac and Tac had remarkable effects on the swelling degree and the dehydration performance of PVA/PAN composite membrane. With increasing the contents of cross-linking agents, the swelling degree of PVA membrane in EAc aqueous solution decreased. As a result, the separation factor of PVA/PAN composite membrane increased, while the permeation flux decreased. The PV performance of cross-linked PVA/PAN composite membrane was investigated at different feed temperatures and feed water concentrations. For PVA/PAN composite membrane cross-linked by Tac with Tac/PVA mass ratio of 0.2, its PV performance for 2 wt.% water of EAc solution at 40 °C was as follows: the separation factor and the permeation flux were 7270 and 34.5 g m −2 h −1, respectively. In addition, the cross-linked PVA/PAN composite membrane also exhibited good dehydration performance for EAc/EtOH/H2O ternary solution. Acknowledgments The authors acknowledge Education Department of Zhejiang Province (GD10071010045), the National Key Fundamental Research Development Plan (“973” Plan, No.2003CB615705) and Chemistry & Chemical Technology Research Center Plan of Shanghai Huayi Group Company (A200-8608) for giving financial supports in this project. References

Fig. 10. The permeation fluxes of EAc/EtOH/H2O mixtures vs. the feed temperature for PVA/PAN composite membrane.

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